Unfortunately, there is no way of doing a bulk download of all your Proxy Bundles. You would either need to do these one at a time through the UI or use a management API Call as below, however, in the API call also, you would need to specify the version number of the bundle you would like to download. You could potentially write a script which will accomplish a bulk download using the call below
The way it works: It queries the org for the list of configured API proxies, then for each proxy, queries for the set of revisions, and then, for each combination of Api proxy + Revision, calls that export API to get the zipped bundle.